As portable electronic devices such as video cameras, cellular phones, notebook computers, etc., become more lightweight and have increasingly improved performance, research into batteries used as power supplies for such portable devices is being conducted. In particular, rechargeable lithium secondary batteries are being actively researched as they have three times the energy density per unit weight compared to conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and can be rapidly charged.
In general, since a lithium battery is generally driven at a high operating voltage, a conventional aqueous electrolytic solution cannot be used. This is because lithium contained in an anode and an aqueous solution vigorously react with each other. Thus, an organic electrolytic solution in which a lithium salt is dissolved in an organic solvent is generally used as the electrolyte in a lithium battery. Such organic solvents should generally have high ionic conductivity, a high dielectric constant and low viscosity. However, since it is difficult to obtain a single organic solvent satisfying all these requirements, a mixed solvent may be used including, for example, an organic solvent with a high dielectric constant and an organic solvent with a low viscosity.
When using a carbonate-based polar nonaqueous solvent, the carbon of an anode and an electrolyte in the lithium secondary battery react with each other during the initial charging to form a passivation layer such as a solid electrolyte interface (SEI) film on a negative electrode surface by an irreversible reaction. The SEI film enables the battery to be stably charged and discharged without further decomposition of the electrolytic solution (J. Power Sources, 51 (1994), 79-104). The SEI film also acts as an ion tunnel through which only lithium ions pass, and prevents cointercalation of an organic solvent, which solvates the lithium ions and moves with the lithium ions into a carbon anode, thereby preventing a breakdown of the anode structure.
However, since a high voltage of 4 V or greater is repeatedly produced during charging and discharging, the SEI film prepared with the polar solvent and lithium salt cannot function ideally. The SEI film can develop cracks, a reduction reaction occurs, insoluble salts form inside and outside the anode, gas is generated, and thus cracks occur in the anode structure. The electronic connection deteriorates because of such cracks in the structure of the anode. Therefore, the inner resistance increases and the capacity of the battery decreases. Since the solvent decomposes and the amount of the electrolyte decreases, the electrolyte in the battery is depleted and sufficient ions cannot be transferred.